Metal oxide semiconductor field effect transistors (MOSFETs) built on a semiconductor-on-insulator (SOI) substrate in general offer advantages over a MOSFET with comparable dimensions that is built on a bulk substrate by providing a higher on-current and lower parasitic capacitance between the body and other MOSFET components. However, a MOSFET built on an SOI substrate tends to have less consistency in the device operation due to “history effect,” or “floating body effect,” in which the potential of the body, and subsequently, the timing of the turn-on and the on-current of the SOI MOSFET are dependent on the past history of the SOI MOSFET. Furthermore, the level of leakage current also depends on the voltage of the floating body, which poses a challenge in the design of a low power SOI MOSFET.
The body of an SOI MOSFET stores charge which is dependent on the history of the device, hence becoming a “floating” body. As such, SOI MOSFETs exhibit threshold voltages which are difficult to anticipate and control, and which vary in time. The body charge storage effects result in dynamic sub-threshold voltage (sub-Vt) leakage and threshold voltage (Vt) mismatch among geometrically identical adjacent devices.
One exemplary semiconductor device in which the floating body effects in SOI MOSFETs are particularly a concern is a static random access memory (SRAM) cell, in which Vt matching is extremely important as operating voltages continue to scale down. The floating body also poses leakage problems for pass gate devices. Another exemplary semiconductor device in which the floating body effects are a concern is a stacked SOI MOSFET structure, as used in logic gates, in which the conductive state of SOI MOSFET devices higher up in the stack are strongly influenced by stored body charge, resulting in reduced gate-to-source voltage (Vgs) overdrive available to these devices. Yet other exemplary semiconductor device in which control of the floating body effects is critical is a sense amplifier for SRAM circuits and current drivers in a current mirror circuit.
While floating body effects may be reduced by increasing leakage current from the body to the source and drain regions, for example, by increasing halo implant dose, such an approach may increase leakage current between the drain and the source, thus increasing the off-current. Ideally, body to drain leakage should be maintained at a minimum level, while body to source leakage should be controlled at a high enough level to suppress the floating body effects by leaking the charge formed in the floating body region.
In view of the above, there exists a need for a semiconductor-on-insulator (SOI) metal oxide semiconductor filed effect transistor (MOSFET) exhibiting reduced floating body effects and having a low off-current, and methods of manufacturing such an SOI MOSFET.